Mounting evidence demonstrates that costimulation delivered by the B7 family regulates T cell proliferation, cytokine secretion, and effector function. T cell receptor (TCR) signaling, in the absence of B7 mediated costimulation, results in the induction of anergy. Although differentially expressed on populations of APCs, both B7- 1 and B7-2 appear to be equally capable of preventing the induction of anergy since both signal through the T cell surface molecule, CD28. CTLA4 is a second T cell surface, lineage restricted receptor for both B7-1 and B7-2. CTLA4 differs from CD28 in that it is not constitutively expressed on resting T cells, has a higher affinity for both B7-1 and B7-2, and does not appear to deliver a positive signal to T cells. An immunoglobulin fusion protein of CTLA4, termed CTLA4-Ig, is a highly efficient reagent to inhibit B7 family mediated costimulation. Murine models demonstrate that reagents which block the B7:CD28 pathway can induce transplantation tolerance and/or control autoimmunity whereas expression or overexpression of B7 family members can induce or amplify immunity to infection or tumors. Notwithstanding the importance of these two CD28/CTLA4 counter-receptors, we and others have increasing evidence that additional, functionally distinct CD28/CTLA4 counter-receptors exist. To date, the molecular structure and function of these molecules is presently unknown. Therefore, the goal of this proposal is to attempt to characterize the structure and function of these molecules. These alternative CD28/CTLA4 counter- receptors appear to be differentially expressed on professional and non- professional APCs and depending the nature of the molecule can provide either costimulatory, inhibitory, or even apoptotic signals to T cells: Because of the heterogeneity of their expression and function, we believe these molecules are likely to be as important as B7- l and B7-2 in the regulation of the immune response. To attempt to understand the function of these molecules, we propose 4 SPECIFIC AIMS: First, to attempt to characterize molecularly and functionally the non-B7-l, non-B7-2 CTLA4-Ig binding ligand(s) that mediate Ag specific apoptosis. Second, to attempt to characterize molecularly. and functionally the non-B7- l, non-B7-2 costimulatory ligand(s) expressed on keratinocytes. Third, to attempt to characterize molecularly and functionally the BBl molecule expressed on activated B cells, BALM4 cells, and other BBl+B7-l- cells. Fourth, to clone and characterize additional novel CD28/CTLA4 binding ligands from libraries prepared from B7-l, B7-2 deficient mice in an attempt to discover members of this family with distinct functions. Once these molecules are in hand, we plan to determine their contribution to immune regulation in human in vitro systems, in murine models, and finally, to determine whether they are involved in the pathogenesis of human disease. Ultimately, they may provide novel therapeutic reagents in the fields of transplantation, tumor immunity, and autoimmunity.